In my recently issued U.S. Pat. No. 4,485,284, an apparatus and process is disclosed for enabling the measurement of moisture in a sample cell by use of microwave energy. As shown therein, the microwave energy is transmitted from a transmitting antenna, passed through the sample cell containing the material of which the moisture content is to be determined, and to a receiving antenna.
In this and similar devices, the microwave antenna for transmission and reception of the microwave energy generates a divergent beam with a spherical wavefront. The materials to be measured by the microwave signal means are in relatively close proximity to the microwave antennas in what is commonly referred to in the art as the "near field" of an antenna. The dielectric properties of materials in the near field of an antenna are known to influence the antenna beam pattern.
The dielectric properties of water are significantly different from most other materials. Thus, changes in moisture content significantly alter the dielectric properties of the materials to be measured. However, this also results in an altered antenna beam pattern in such moisture detectors using microwave signal means. In measuring homogeneous materials where the moisture is uniformly distributed within the sample material, the changes in antenna pattern due to moisture changes become part of the calibration with no detrimental effects in most cases. Inhomogeneous materials containing varying densities, lumps, air voids, or nonuniform distribution of moisture within the sample are much more difficult to measure using a conventional microwave beam. The effect of material nonuniformities and resultant discontinuities in the antenna pattern due to nonuniform dielectric properties create inaccuracies in measurement. The magnitude of the discontinuity effect due to nonuniform dielectric properties depends on where the discontinuity is located within the microwave beam pattern and the angle of incidence of the divergent microwave beam and the discontinuity.